1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, is concerned with an improved water displacer rod containing hollow sealed pellets which prevent the rod from filling with reactor coolant in the event of rod failure.
2. Description of the Prior Art
In the conventional designs of pressurized water reactors, an excessive amount of reactivity is designed into the reactor core at start-up so that as the reactivity is depleted over the life of the core there will still be sufficient reactivity to sustain core operation over a long period of time. However, since an excessive amount of reactivity is designed into the reactor core at the beginning of the core life, steps must be taken at that time to properly control it.
One technique to control reactivity is to produce an initial spectral shift which has the effect of increasing the epithermal (low reactivity) part of the neutron spectrum at the expense of the thermal (high reactivity) part. This results in production of fewer thermal neutrons and decreased fission. Then, as fission decreases during extended reactor operation, a reverse shift back to the thermal part of the neutron spectrum at the expense of the epithermal part is undertaken.
Such control technique is primarily accomplished through the use of displacer rods. As the name implies, these rods are placed in the core to initially displace some of the moderator water therein and decrease the reactivity. Then, at some point during the core cycle as reactivity is consumed, the displacement associated with these rods is removed from the core so that the amount of moderation and therewith level of reactivity in the core are increased. One approach for removing moderator water displacement is described in a U.S. patent application entitled "An Improved Water Displacer Rod Spider Assembly for a Nuclear Reactor Fuel Assembly" by Trevor A. Francis, filed Mar. 30, 1984 and assigned U.S. Ser. No. 595,154.
Several constructions of a water displacer rod are illustrated and described in U.S. Pat. No. 4,432,934 to Gjertsen et al. Referring to FIGS. 12 and 13 of this patent, the displacer rod includes an upper first tube composed, for instance, of stainless steel and a lower second tube composed, for example, of Zircaloy-4, with the tubes being interconnected by a mid extension in the for of a rod of solid Zircaloy-4. The upper first tube is sealed at its upper end by a top end plug capable of being threadably attached to a spider, while the lower second tube is sealed at its lower end by a bottom end plug which can be bullet shaped to aid in insertion of the rod into a guide thimble within one of the fuel assemblies of the reactor. In one construction of the displacer rod shown in FIG. 12, the second tube is thin walled and contains a stack of annular pellets extending from the bottom end plug to the mid rod extension which provides structural support for the tube to prevent its collapse under reactor operating pressure. In the alternative, the pellets can be of a solid instead of a hollow construction. As another alternative construction, the second tube can be thick walled and pressurized with an inert gas to prevent collapse of the rod under reactor pressure. In still another construction, the rod contains a spring instead of the stack of pellets. In any event, the construction of the displacer rod is such that it provides a low neutron absorbing rod that is capable of displacing reactor coolant-moderator when inserted into a fuel assembly.
While all of the above-cited prior constructions of the water displacer rod perform reasonably well and generally achieve their objectives under the range of operating conditions for which they were designed, a need exists for improvement in the design of the displacer rod so as to increase its general utility in fuel assemblies in a variety of slightly deformed structural states and to facilitate even more efficient use of nuclear fuel in the reactor core through employment of the spectral shift principle.